eso8804-en-ie — Science Release

A Picture of Comet Halley at 1250 Million Kilometres

8 July 1988

A unique picture of Comet Halley has just been obtained by three astronomers with a telescope at the ESO La Silla observatory. An exposure time of almost 12 hours was necessary to show the structure of the famous comet in some detail at the present, very large distance of 1250 million kilometres.

The point-like image of the comet's nucleus is clearly visible, although it is more than 6 million times fainter than what can be seen with the unaided eye. For the first time, it has also been possible to get a detailed view of a large, asymmetric dust cloud around a comet so far from the Sun. It appears that Comet Halley is still actively dispensing dust into the surrounding space.

A B/W version of the picture, suitable for reproduction, accompanies this Press Release. A false-colour version with more details is available on request.

Ground-based observations continue

Comet Halley made headlines when it was visited by no less than five spacecraft in March 1986. But whereas the space-based studies of Comet Halley lasted a few weeks only, observations with ground-based telescopes still continue. Thanks to extraordinary efforts by astronomers all over the world, an incredible wealth of observations was obtained when the comet was near the Sun in 1985-86. For the fullest possible understanding of the cometary processes, it is now important to follow the comet as far as possible on its way back towards the icy depths of the outer solar system.

However, the comet becomes fainter and fainter as it recedes from the Sun. Since late 1986 it has only been visible as a fuzzy point of light. What we see is the reflection of sunlight from a cloud of dust, that still remains around the nucleus. And now, more than two years after the perihelion passage, the comet is so faint that it can only be observed with the largest telescopes under excellent conditions.

The ESO picture

Comet Halley was observed during 19 nights in April and May 1988 with the Danish 1.5 m telescope at the ESO La Silla observatory. A highly sensitive CCD-camera was used to obtain more than 60 images of the comet. In order to detect the faint light from the moving comet, the telescope was set to follow the comet's motion. Fortunately, the observations benefitted from very good atmospheric conditions throughout this period.

Extensive image processing was carried out with the ESO IHAP system, including “removal" of stars and galaxies in all frames. By coadding about 50 exposures, with a total exposure time of 11 hours 35 minutes, it is now possible to present a picture of the Comet Halley as it appears at a distance of 1250 million kilometres from the Sun (almost as far away as the planet Saturn). No picture with so much detail has ever been obtained of any comet so far from the Sun.

The nucleus (which is too small to be resolved at this distance) is clearly seen as a small, bright point. The visual magnitude was determined as 23.1 with variations from night to night by a factor of 2, reflecting the rotation of the avocado-shaped nucleus. It is located off-center in a relatively bright, asymmetric region, called the “inner coma"; it measures about 20 arseconds across, corresponding to 120.000 kilometres. Further out, a larger, “outer coma" of elliptical shape can be distinguished; it measures at least 45 arcseconds (300.000 kilometres) across and is possibly significantly larger. The faintest contour shown is about 27 mag per square arcsecond, or 100 times fainter than the light from the night sky. Summing all of the light, the comet's total magnitude is found to be around 17.

From the shape and density of the inner coma, it would appear that dust is still being released from the nucleus, even at this large distance. By the action of the solar wind, it is pushed away from the nucleus and slowly dissipates into the surrounding interplanetary space. The presence of such a large and faint, outer coma around a comet at this distance has not been detected before; this was only possible because of the very long exposure time.

It is expected that further observations will be made at ESO in early 1989, when Comet Halley will be more than 1500 million km away and also in 1990 (1900 million km) with the 3.6 m telescope or the new 3.5 m New Technology Telescope. However, at that time the comet will be significantly fainter than now and any future picture will show less detail. Halley crosses the orbit of Uranus in April 1994, that of Neptune in February 2006 and reaches the most distant point in its orbit in April 2024, 5300 million km from the Sun.

More information

A picture has been obtained by three astronomers:H.E.Jørgensen and P.Kjærgaard (Copenhagen University Observatory); R.M.West (ESO).